Spark 1490 Add kerberos support to the HistoryServer

Here I've added the ability for the History server to login from a kerberos keytab file so that the history server can be run as a super user and stay up for along period of time while reading the history files from HDFS.

Author: Thomas Graves <tgraves@apache.org>

Closes #513 from tgravescs/SPARK-1490 and squashes the following commits:

e204a99 [Thomas Graves] remove extra logging
5418daa [Thomas Graves] fix typo in config
0076b99 [Thomas Graves] Update docs
4d76545 [Thomas Graves] SPARK-1490 Add kerberos support to the HistoryServer